Devices for conversion of heat energy to electrical energy by electrochemical expansion of a working substance across a solid electrolyte have been described heretofore. See, for example, U.S. Pat. Nos. 3,458,356 and 3,511,715.
Devices, using, for example, alkali metal as a working fluid, generally comprise a closed container separated into two different pressure regions by solid electrolyte. In the higher pressure region, alkali metal is in fluid contact with the electrolyte or electrode in electrical contact therewith. In the lower pressure region, a porous electrode is in electrical contact with the solid electrolyte. The vapor pressure differential between the regions causes migration of alkali metal cations through the solid electrolyte with concomitant loss of electrons to an external circuit. These electrons flow through the external circuit and recombine with cations passing out of the solid electrolyte at the porous electrode. Neutral alkali metal evaporates from the surface of the porous electrode and travels to a cooler collection zone for condensation as to a liquid and return to the higher pressure region whereby the cycle is completed.
Operation of such above described prior art devices was heretofore thought to be optimal when there is a maximum pressure difference between the higher and lower pressure regions. As a consequence, the vapor pressure of alkali metal in the lower pressure region was normally desired to be very low, i.e., in the range of millitorrs. At this pressure range, the pressure differential between the higher pressure region and lower pressure region would be greatest, leading to greatest predicated voltage outputs. Moreover, such prior art devices utilize a collection zone for alkali metal vapor which consists of cooled wall or walls of the lower pressure region onto which alkali metal condenses and drains for return to the higher pressure region. Heat radiation, however, deleteriously follows the path of this alkali metal to the cooled walls and to date effective means for reducing these radiative heat losses is not believed to have been accomplished.
In devices and method of this invention, vapor pressure in the lower pressure region is optimally maintained at a level sufficient to permit hydrodynamic flow, e.g., about 0.05 torr or higher, of the alkali metal vapor after its vaporization from the surface of the porous electrode. The alkali metal vapor in the lower pressure region can then pass hydrodynamically through shaped openings, e.g., slits, orifices, nozzles, which constrict the flow of vapor while minimizing heat radiation loss to the cooling condensers.
Advantageously, a plurality of high pressure regions now can be incorporated in accordance with this invention into one device thereby leading to greater predicted power outputs for the devices as compared to prior art devices and methods. Further, series electrical connection within the device can reduce conduction losses. Still further, fast hydrodynamic flow of alkali metal vapor through shaped openings can eliminate radiative heat losses as a matter of practical concern.